1. Technical Field of the Invention
The present invention relates to a vehicular system that is for use in a motor vehicle, such as a passenger car or a truck, and is configured to control the rotational speed of an engine and the gear ratio of a transmission according to the State of Charge (SOC) of a battery.
2. Description of the Related Art
There is known, for example from Japanese Patent First Publication No. H5-124460, a powertrain control apparatus for use in a motor vehicle. This apparatus is configured so that when the SOC of a battery drops below a predetermined threshold, the apparatus changes the gear ratio of a transmission so as to increase the rotational speed of the engine, and decreases the amount of fuel fed to the engine so as to decrease the output power of the engine. With such a configuration, when the electric power consumption of the vehicle increases, the apparatus can increase the rotational speed of the engine, thereby increasing the amount of the electric power generated by an automotive alternator which charges the battery. As a result, the SOC of the battery can be kept at a high level, thus allowing the battery to have a low capacity and size.
However, with the above configuration, when the power generation torque of the alternator increases with the amount of the electric power generated by the alternator, but the output power of the engine is not suitably decreased, a rapid increase or decrease will be caused in the rotational speed of the engine. As a result, the running performance of the vehicle will be lowered, thus inconveniencing the driver of the vehicle.
Moreover, in recent years, Continuously Variable Transmissions (CVTs) have come to be used in vehicular systems which are not of a hybrid type and employ a general engine. Since the gear ratio of a CVT can be steplessly or continuously changed, it is possible to run an engine connected to the CVT in a low rotational speed region where the efficiency of the engine is high.
However, as shown in FIG. 6, when the rotational speed of the engine decreases, for example from A to B, the output current of an alternator driven by the engine also decreases. As a result, a battery, which is configured to be charged by the alternator, can no longer be charged by the alternator, thus eventually running down. In addition, in FIG. 6, the CVT region denotes the region of the rotational speed of the engine where a CVT can be used; the AT region denotes the region of the rotational speed of the engine where an automatic transmission can be used.